The white residue is caused by moisture in your oil. How it got there is the question... you say it is just a little bit, which could just be an indicator that the car went on a lot of short trips and the oil didn't get fully warmed up (which would burn off any moisture in the system).
If there were a significant amount of mayonnaise on the cap, then I'd be more worried. Something like that would indicate more moisture (most likely coolant) getting into the oil, which typically points to a head gasket. Though on the Boxster it may be different... I'll let somebody with more Boxster cooling system experience chime in.
Good luck...
